Mention brand specific last digits expectations

Author: ugexe

CHANGELOG.md

@@ -27,10 +27,16 @@ CHANGELOG
   `lastDigits`/`last4digits` also now supports two digit values in
   addition to the previous four digit values.
 * Eight digit `/credit_card/issuerIdNumber` inputs are now supported in
-  addition to the previously accepted six digit `issuerIdNumber`. If you
-  send six digits for the `issuerIdNumber`, you should send the last four
-  digits for `lastDigits`. If you send eight digits for `issuerIdNumber`
-  you should send the last two digits for `lastDigits`.
+  addition to the previously accepted six digit `issuerIdNumber`. In most
+  cases, you should send the last four digits for `lastDigits`. If you send
+  an `issuerIdNumber` that contains an eight digit IIN, and if the credit
+  card brand is not one of the following, you should send the last two digits
+  for `lastDigits`:
+  * `Discover`
+  * `JCB`
+  * `Mastercard`
+  * `UnionPay`
+  * `Visa`
 
 4.3.0 (2021-08-31)
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